Electrical power generation and distribution systems typically employ various means for protecting both the distribution equipment (e.g., generators, controllers, distribution wiring and feeders, etc.) and the utilization equipment connected thereto (e.g., computers, lighting systems, fans, motors, control units for other systems, etc.) from faults which could otherwise cause extensive damage. Protective functions such as over voltage, under voltage, over frequency, under frequency, etc., are designed so that the utilization equipment are not damaged by a failure within the controls or equipment which regulates the power produced by the electrical system. At the same time, functions such as over current, shorted generator diode, differential current, etc., protect the generation equipment from damage which may result as a result of failures within the generators, along the distribution feeders, or even within the utilization equipment, e.g. short circuits, open phase wires, etc.
Of these protective functions, the differential current protection is particularly important. A differential current fault includes short circuits, high impedance to ground faults, and line-to-line faults which occur somewhere along the feeders between the generator and the utilization equipment. These faults are of particular importance because at least some of the current injected into the feeders by the generator is going to a fault and not to the loads. This differential current may result in fire or other damage. For many ground based electrical systems, protection against this type of fault is provided by installing circuit breakers or like devices at the input to the distribution system which go open circuit, or "blow", when one of these faults occur due to the substantially increased current flow along the feeders. The typical time for actuation of one of these devices is related to the amount of current flowing, and may be as long as 10 seconds depending on the application. For some high impedance faults, however, the threshold current level may never be reached, and the fault could exist indefinitely, thus reducing the efficiency and safety of the system.
For airborne electrical systems, however, the electrical feeders often traverse fuel tanks and pass through volatile areas. Allowing a short circuit or other differential fault to last for such time frames as are allowed in ground based systems could result in fire, explosion, and crash of the aircraft. To protect against such disasters, airborne electric power generation and distribution systems utilize a much more sophisticated approach to detect and isolate these types of faults. The differential current protection system used on aircraft works under the simple principle that what goes in, must come out. For electrical systems the principle states that the amount of current injected into a feeder must equal the amount of current supplied to the loads (the utilization equipment). The amount of current generated at the source is monitored and compared to the amount of current supplied to the loads. Any difference between these two values indicates that a fault exists somewhere in between. Upon detection of such a fault, the system controller can isolate the fault and reconfigure the system to compensate within 20 to 120 milliseconds.
As shown in FIG. 1, the input monitoring devices 100 and the output monitoring devices 102, e.g. current transformers, define a differential protection (DP) zone. Protected within this DP zone for a typical 400 Hz system are the generator windings 104, main line contactors 106, terminal connectors, etc. Inclusion of the entire system from the generator 108 to the loads 110 is possible because the current generated by the generator 108 is delivered to the loads 110 without conversion, i.e. the power generated is 115 Vac, 400 Hz and the power used is 115 Vac, 400 Hz.
As shown in FIG. 2, another topology system exists, however, wherein the power generated is converted prior to delivery to the loads. This type of system makes it difficult to compare the current generated by the generator to that which is ultimately delivered to the loads due to the varying efficiencies of the conversion equipment under the various loading conditions. For this reason, DP zones are established for each stage of power conversion. A first DP zone exists from the generator 112 to the converter 114, a second from the converter 114 to the loads 110. Unfortunately, these DP zones do not overlap, and therefore cannot sense faults within the converter 114.
A popular converter topology used for airborne applications is known as a dc link system. This topology transforms the input variable frequency electric power generated by the wild frequency generator 112 into dc electric power via an input bridge rectifier 116. This dc electric power is then input via a dc link 118 to a switching network 120 which produces constant frequency electric power for use by the utilization equipment 110. Although the sophisticated control of the switches can detect and isolate faults which occur at its stage of conversion (de to constant frequency ac conversion), no system of protection could detect and isolated a fault occurring at the other stage of conversion within the converter (wild frequency ac to dc conversion). The result is that a short circuit across one of the rectifiers in the ac to dc conversion stage could go undetected, depending on generator speed and loading conditions, resulting in reduced safety and efficiency, and potential damage to the converter as well as to the aircraft. The likelyhood of detection is reduced at high generator speeds and low loads, which is the normal operating mode for back-up and emergency power systems. The present invention is directed to overcoming this problem.